Control metal reflection color with PR Shader and an Albedo input texture
The metalness workflow uses an Albedo map to define both the diffuse and reflectivity content.


for insulators the diffuse is taken from albedo, reflectivity color is white and the amount reflected is small (about 4%).
for metals the diffuse is black, reflectivity color is taken from albedo and amount reflected is 100%.


The metalness map is then used to define how much the material is an insulator or a metal and so how the albedo color is used.

It is setup like this to save texture space, which is important to game engines and to render engines alike. In the specular workflow the diffuse and reflectivity colors are set explicitly with two unique texture maps. You can make PBR materials with both approaches, more on this is nicely written on Marmoset's site.